Yesterday, I donwloaded and installed Dragon Go! onto my friends iPhone (I don't have one yet) and onto my iPad. From what I understand, the makers of Dragon are collaborating with Apple to develop Siri...
Not sure why others are having trouble finding Dragon Go! in the US app store. It's an iPhone app and runs on the iPad 2. Downloaded and tried it. It's very similar, in fact, to some voice command apps I have on an Android phone. Works well, but it's very limited compared to what Siri amounts to (at least from the initial reviews.)
Basically, the Dragon Go! app is integrated with a limited set of other apps you may have on your phone. i.e. Accu Weather, your iPod music app, Yelp!, etc. About a dozen apps as far as I can tell. Ask the right question and Dragon Go! automatically opens the appropriate app with the parameter(s) you've spoken. I'd assume that Siri operates in a similar fashion but with more comprehensive integration with various apps on the phone including contacts, calendar, etc. (e.g. "Remind me when it's Dad's birthday.)
My limited testing suggests it has some bugs (at least on the iPad). For example, if it's used to play a song and then closed, using some other music apps (RadioBox) will no longer play in the background once closed. Pandora, however, continues to operate correctly.
All in all, the app appears to operate relatively well, at least in a quiet room with carefully spoken commands. As noted, though, it's neither as powerful as Siri is expected to be nor as powerful as some other existing Android apps.